1. Field of the Invention
The present invention relates to a rotation table, and particularly, to a rotation table with a clamp torque measurement unit.
2. Description of the Related Art
In a rotation table of a machine tool, a clamp operation is performed during an index operation so as to hold a rotation shaft. When operations such as an unclamp operation, an index operation, and a clamp operation are repeated, a clamp mechanism is degraded, and hence there is a case in which a necessary clamp torque may not be exhibited. In a case where the necessary clamp torque may not be exhibited, when a rotation load is applied to the rotation shaft, the rotation shaft may not be held, and hence a slip occurs. Thus, the machine tool stops with an alarm generated by the slip, and a workpiece which is being machined becomes a defective product.
Accordingly, there is a need to provide means for periodically measuring the clamp torque of the clamp mechanism so that a deviation in rotation shaft does not occur due to a decrease in clamp torque during the machining. Further, if the clamp torque can be temporarily increased even when a decrease in clamp torque is checked, the machine tool may be used continuously, and hence the sudden stop of the machine tool may be prevented. That is, the maintenance timing may be extended.
A rotation table with a clamp state checking means disclosed in Japanese Patent Application Laid-Open No. 2009-248242 includes oscillation instruction means for repeatedly instructing a normal rotation and a reverse rotation of a direct drive motor, positional deviation information calculation means for calculating positional deviation information based on position information output from a position detector, and determination means for determining a clamp state based on the positional deviation information output from the positional deviation information calculation means. The rotation table causes a slight oscillation operation in the rotation shaft due to a change in torque, and determines whether the clamp operation is completed or not based on the fluctuation range of positional deviation that occurs at that time. However, it should be noted that the rotation table does not accurately measure the clamp torque.
As described above, in the above-described known rotation table with clamp state checking means, the clamp torque needs to be accurately measured. However, as a method of measuring the clamp torque, there is known a method of applying a torque to the rotation shaft from the outside in the rotation direction and measuring the rotational displacement at that time. When the rotation torque increases, the slip occurs at the stage in which the rotation torque becomes larger than the clamp torque, and hence the displacement in the rotation direction increases. Here, it is possible to measure the clamp torque by measuring the changing point of the displacement and the rotation torque at that time.
However, in this method, since a jig for applying the rotation torque from the outside needs to be attached to the rotation table, there is a need to remove a machining jig or a workpiece on the rotation table once. Further, there is a need to perform an operation in which a measurement jig is removed after the measurement and the workpiece and the machining jig are set up again. Here, it is not realistic to remove the workpiece or the machining jig for the measurement of the clamp torque.